1. Field of the Invention
The invention relates to a process and to a circuit arrangement for electrically monitoring the overload of an electric motor drive.
2. Description of the Related Art
Electric motor drives are in very widespread use, for example for driving electric tools, domestic appliances, office machines, pumps, conveyor systems, positioning devices, electric railways etc. A differentiation is drawn between direct-current, alternating-current and three-phase motors as well as stepping motors, which are used either in the continuous-current mode or in the pulsed-current mode.
A common feature of all applications is that the electric motor drive must move an object which can have a great weight, depending on the application (for example the conveyed material and the conveyor belt in the case of electrically driven conveyor systems, or the electrically movable gantry of a gantry measurement machine). In order to move the object, the electric motor drive must consume electric power, the so-called real power. The heavier the object to be moved, the greater is the load on the motor and thus the real power of the motor. A major change in the real power occurs at the time when the load is connected, that is to say when the electric motor drive is mechanically connected to the object to be moved (for example by a coupling between the two). There is likewise a load connection when, for example, an electrically driven milling tool touches the workpiece to be machined.
After the electric motor drive has been switched on, its rotation speed rises continuously until the nominal rotation speed which is specified for the respective application (i.e., a predetermined rotation speed for the respective application) is reached. The nominal rotation speed when the load is not connected is called the no-load rotation speed. When the load is connected, the nominal rotation speed falls from the value of the no-load rotation speed in proportion to the load. The time between switching on the electric motor drive and reaching the nominal rotation speed is called the starting phase, and the time after that is called continuous operation.
In continuous operation, the load capacity of an electric motor drive is limited by its heating. The actual real power in continuous operation is called the nominal power, the maximum permissible nominal power being the rated power. Electric motor drives may be briefly loaded beyond their rated power.
The real power rises very quickly from zero during the starting phase, that is to say after the electric motor drive has been switched on, to a peak value which is considerably above the rated power, and does not fall to the value of the nominal power until the nominal rotation speed is reached. The peak value of the real power occurs because the electric motor drive initially has to overcome its own mechanical inertia and the real power consumption is limited in the starting phase only by the winding resistance and inductance of the electric motor drive. For an electric motor drive which is subject to load, this peak value is above that of the unloaded electric motor drive.
A process of the type mentioned initially is known for avoiding overheating of an electric motor drive itself, or else, for example, for identifying tool wear and tool fracture on machine tools which are driven by electric motors. It is suitable for monitoring the overload of electric motor drives (direct current, alternating current or three phase) in continuous-current mode, whose load is not connected until the nominal rotation speed is reached, such as machine tools, for example. In this case, use is made of the fact that the time profile of the real power curve is constant after the nominal rotation speed has been reached and rises to a level proportional to the load when the load is connected. If an overload occurs, the instantaneous real power rises suddenly. A comparison of the instantaneous real power with a power warning threshold which represents the overload state and is itself time-dependent for loads which vary with time, thus indicates that a critical state has been reached. A switching and/or warning signal is initiated when the power warning threshold is exceeded.
For monitoring, the current consumption is measured and the instantaneous real power is determined from it. In the case of electric motor drives which are operated using direct current, the current measurement is carried out by means of a Hall sensor, and the current measurement is carried out by means of a current transformer in the case of electric motor drives which are operated using alternating current. The current measurement can be carried out continuously.
The actual monitoring with threshold value comparison is, however, not switched on in the case of the known process until the starting phase of the electric motor drive has been completed. This is done at a preselected time after the electric motor drive has been switched on, since the normal time profile of the real power curve is known. However, the switching-on time can also be determined from when the real power reaches a constant value.
The known process therefore does not cover all the operating states of an electric motor drive. This is based on the fact that, in the case of all electric motor drives in the starting phase, the power consumption rises rapidly to a peak value well above the power warning threshold for an overload state and does not fall to a constant value, well below the peak value, thereafter until the nominal rotation speed has been reached. In practice, the described peak value is frequently not reached as a result of the amplifier stages of the power supply device of the electric motor drive being overdriven, but a plateau occurs before reaching the peak value in the time profile of the real power, which plateau is well above the power warning threshold. Its level is determined by the current amplifier limiting. It is thus also not possible to measure additional peak values, which are caused by overloads, in the region of the amplifier limiting. The known process is thus not suitable for monitoring the overload during the starting phase of an electric motor drive.
The known process can thus also not be used for monitoring the overload of pulse-driven electric motor drives or stepping motors since they are operated, for example, in the case of short movement paths or slow movement speeds, below their nominal rotation speed and, in the sense of the present description, thus do not come out of the starting phase.
However, it is desirable to monitor the overload in all the operating states, especially for electric motor drives having a permanently connected load (for example positioning devices), because they can reach an overload state even before reaching the rated rotation speed (for example as a result of mechanical seizure).